1. Technical Field of the Invention
The present invention relates to a semiconductor device that photographs images and outputs digital image data.
2. Prior Art
In recent years, digital still cameras and digital video cameras with over one million pixels have come into use. These digital still cameras and the like are quipped with an area sensor and an A/D converter, and obtain digital image data by digitally converting output signals (analog image signals) of the area sensor by the A/D converter. Digital still cameras and the like used in recent years need an A/D conversion speed of about 20 MHz–30 MHz, and therefore often use pipeline type A/D converters that are capable of performing high-speed A/D conversion. FIG. 6 shows an example of a conventional circuit (digital image photographing circuit) for photographing digital images. Referring to FIG. 6, the digital image photographing circuit 40 is equipped with an area sensor 41 having sensors disposed in matrix, a signal latch section 42 that latches analog image signals output from the sensors within the area sensor 41, a CDS circuit 43 that removes noise components from the analog image signals output from the signal latch section 42, and a pipeline type A/D converter 44 that converts the analog image signals output from the CDS circuit 43 into digital image data.
The digital image photographing circuit 40 shown in FIG. 6 generally uses an image sensor having a CCD (charge coupled device) transmission circuit that can photograph high-quality images with low noise. For A/D conversion, a pipeline type A/D converter is used to photograph moving pictures and take out digital image data at a high speed.
In recent years, digital still cameras and digital video cameras with over one million pixels have come into use. These digital still cameras and the like are equipped with an area sensor and an A/D converter, and obtain digital image data by digitally converting output signals (analog image signals) of the area sensor by the A/D converter. Digital still cameras and the like used in recent years need an A/D conversion speed of about 20 MHz–30 MHz, and therefore often use pipeline type A/D converters that are capable of performing high-speed A/D conversion. FIG. 6 shows an example of a conventional circuit (digital image photographing circuit) for photographing digital images. Referring to FIG. 6, the digital image photographing circuit 40 is equipped with an area sensor 41 having sensors disposed in matrix, a signal latch section 42 that latches analog image signals output from the sensors within the area sensor 41, a CDS circuit 43 that removes noise components from the analog image signals output from the signal latch section 42, and a pipeline type A/D converter 44 that converts the analog image signals output from the CDS circuit 43 into digital image data.
Also, a semiconductor device manufacturing process for CCDs and a semiconductor device manufacturing process for pipeline type A/D converters are completely different from each other, and therefore there is a problem in that the CCDs and pipeline type A/D converter cannot be formed on a single semiconductor chip.
Also, in recent years, CMOS type image sensors that produce higher image quality have been made available. The CMOS type image sensor can be formed with a logic circuit such as an A/D converter on a single semiconductor chip. (Here, the CMOS type image sensor means sensors in general terms that can be formed on a single semiconductor chip using a CMOS manufacturing process which forms A/D converters and digital processing circuits.)
The CMOS type image sensor can be formed on the same semiconductor chip where a pipeline type A/D converter having a large circuit and a large power consumption is formed, but also can be formed on the same semiconductor chip where a column type A/D converter that can achieve a lower power consumption is formed. FIG. 7 shows an example of a semiconductor device using a column type A/D converter. In FIG. 7, a semiconductor device 50 is equipped with an area sensor 51 with CMOS type image sensors disposed in matrix, and a column type A/D converter 52.
FIG. 8 shows an internal structure of the column type A/D converter 52. In FIG. 8, the column type A/D converter 52 includes a counter 53, a sweep signal generation section 54, comparators C11–C1n, and latches L11–L1n.
The counter 53 successively outputs unsigned integer values (0–1023) having a 10-bit width. The counter 53 successively outputs the integer values with a 10-bit width for the following reasons. As described later, the integer values output from the counter 53 are latched by the latches L11–L1n, and output as digital image data. Accordingly, the bit width of the digital image data is the same as the bit width of the integer values output from the counter 53. However, the digital image data with 8-bit width is not sufficient in actual use as high-quality image data, and about 10-bit width is normally required.
The sweep signal generation section 54 outputs sweep signals according to the integer values output from the counter 53. FIG. 9 shows a relationship between the integer values that are output from the counter 53 and potentials of the sweep signals that are output from the sweep signal generation section 54 (i.e., sweep signal output characteristic). In FIG. 9, integer values output from the counter 53 are indicated along a horizontal axis direction, and potentials of the sweep signals that are output from the sweep signal generation section 54 are indicated along a vertical axis direction. As shown in FIG. 9, the sweep signal generation section 54 output potentials that linearly increase with respect to the integer values output from the counter 53. It is noted that, in general, the power supply voltage of a CMOS type image sensor is 3.3V, and an amplitude of an analog image signal that is output from the CMOS type image sensor is about 2 V. Therefore, the sweep signal generation section 54 outputs a 2V sweep signal with an amplitude of (VB)V through (VB+2)V according to the amplitude of the analog image signal that is output from the CMOS type image sensor.
When a column type A/D converter is used, an area sensor and the column type A/D converter can be integrated on a single semiconductor device. However, the column type A/D converter has a problem in that it is difficult to secure the dynamic range. Also, the column type A/D converter has a problem in that the A/D conversion takes a long time because the cycle of the sweep signal in a ramp shape must be made longer in order to achieve a highly accurate A/D conversion.
In view of the above, it is an object of the present invention to provide semiconductor devices in which an area sensor and an A/D converter can be formed on a single semiconductor chip, which is capable of reducing power consumption, converting analog image signals to digital image data with a simple circuit, reducing noise sources, reducing costs, and compressing the bit width of the digital image data while achieving a high dynamic range.